Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
During a write operation, a current is applied to a write element of the head (e.g., a write coil) to create a magnetic field which magnetizes the surface of the disk by orienting the direction of magnetic grains (e.g., horizontally in longitudinal magnetic recording, or vertically in perpendicular magnetic recording). The orientation of the grains exhibits hysteresis thereby generating their own magnetic field when the write magnetic field is removed. During a read operation, a read element of the head (e.g., a magnetoresistive element) transduces the magnetic field emanating from the disk surface into a read signal that is demodulated into an estimated data sequence.
The hysteresis of the magnetic grains is not permanent meaning that over time the grains will orientate into random directions (magnetic entropy) until the magnetic field is no longer sensed reliably (leading to data errors during reproduction). Magnetic entropy may also be precipitated by various factors, such as increasing ambient temperature. That is, at higher temperatures the uniform alignment of the grains will degrade faster. Another factor that precipitates magnetic entropy is a phenomenon referred to as adjacent track interference (ATI) wherein when writing data to a target track, the fringe field from the write element degrades the uniform alignment of the grains recorded in an adjacent track. The fringe field from the write element may also adversely affect a wider range of tracks with respect to the written track, a phenomena referred to as wide area track erasure or WATER. The degrading effect of ATI on the adjacent tracks as well as WATER on the near adjacent tracks compounds over time with each write operation to the target track. Eventually, the magnetic field emanating from the disk surface will deteriorate to the point that the data is no longer recoverable.
To protect the integrity of data within a region of the disk against degradation over time, the data may be refreshed, wherein the data is read from the disk and rewritten back to the disk. The refresh operation may be performed in the background, for example, after a certain number of writes are made within the region of the disk and/or other regions of the disk located near the region of the disk. The disk drive may perform refresh operations for many regions of the disk in the background to protect the data integrity of the disk.
However, when the disk drive is busy handling host commands, the user may experience an undesirable slow down of the command execution time while the disk drive initiates and executes refresh operations in the background, thereby resulting in an undesirable decrease in drive performance. Thus, it is important to determine when a refresh operation for a particular region of the disk is necessary to avoid the undesirable performance penalty associated with performing a premature refresh operation.